Rocky Flats

Rocky Flats stretch for 3 miles north to south, on the east and west sides of the highway. I have seen some references to the “Rocky Flats pediment.” However, it looks more an alluvial fan at the mouth of Coal Creek Canyon.

The highway passes near the center of the NE1/4 SW1/4 Sect 28, T3S, R70W.

Somewhere in this quarter-quarter section are the locations of two trenches dug to explore a small graben of Quaternary age, thought to be related to the Golden fault. Excavations revealed a complex geologic picture suggesting two periods of fault rupture with a total of 5.5 m of displacement. Dating of zircons and glass shards from an ash deposited in the graben and disrupted by faulting yielded dates of 0.6 to 0.7 million years, suggesting faulting occurred more recently than those dates (Kirkham, 1977).

Looking to the north and south,
we can see the distinctive hogback of the Dakota Sandstone at
both ends of the valley. But why is this hogback missing at the
valley center? The answer is that the Golden Fault, a large,
Laramide thrust fault, weaves along the valley just to the east of
the mountain front. Here, the fault has displaced nearly 8,000 feet
of the Paleozoic and Mesozoic, sedimentary section (Van Horn,
1976). The trace of the fault can be seen as it cuts up the side of
the first large alluvial terrace to the south of Clear Creek. There is
a distinct break in the vegetation between the Fountain Formation
to the west (covered with mountain mahogany shrubs) and
the Pierre Shale to the east (covered with grasses).

Looking directly west, we see a road cut for State Highway
93. This portion of the highway was first constructed in
1991. A small landslide formed in the west (opposite) face of the
cut shortly thereafter, and the resulting toe bulge closed the southbound
lanes of the new highway. Within a year, the landslide had
developed a rim of head scarps with up to 12 feet of vertical slippage,
and had captured the surface flow of a small stream in
Magpie Gulch. Early efforts to drain the landslide were unsuccessful.
A full geologic investigation subsequently revealed that
the landslide sits directly atop the Golden Fault, with low-permeability
Pierre Shale on the eastern side and a wedge of fractured,
permeable Fountain Formation on the western side. In 1994,
three lines of rock anchors (about 40 anchors in total) were
installed across the landslide. The Magpie Gulch stream was
piped across the head scarps, longer horizontal drains were
installed, and a remote data-logging unit was set up. To date, this
mitigative effort appears to have been successful. The combined
maintenance and mitigation operations for this incident are
reported to have cost about 3 million dollars.

The houses to the north of the landslide were built shortly
after the landslide was mitigated and all signs of its existence had
been “erased” from view. One can only imagine the concern of
these residents had the landslide been active a few years later.
This is a good illustration of the often short-term public memory
of geologic hazards.